The present invention relates generally to the field of multiplexers and, more specifically, to the field of millimeter wave multiplexers constructed in the stripline, microstrip and suspended substrate media.
The most common method of multiplexing involves the utilization of a series of bandpass and bandstop filters connected as illustrated in FIG. 1. In FIG. 1 a triplexer 12 that is intended to operate in the 75 to 110 GHz band receives its input on input transmission line 14. The triplexer 12 is comprised of two bandpass filters 16 and 18 operating in the 75 to 98 and 75 to 86 GHz bands, respectively. A first bandstop filter 20 is connected between the input and bandpass filter 16 and a second bandstop filter 22 is connected between the bandpass filter 16 and 18. The bandstop filter 20 rejects signals between the 75 to 98 GHz band and therefore passes signals in the 98 to 110 GHz band. The bandstop filter 22 rejects signals in the 75 to 86 GHz band and therefore passes signals in the 76 to 98 GHz band. For a 75 to 110 GHz input signal, therefore, the output signal components in the 98 to 110 GHz band appear primarily on output 24. Output signals in the 76 to 98 GHz band appear primarily on output 26 and output signals in the 75 to 86 GHz band appear primarily on output 28. The filters 16, 18, 20 and 22 are generally designed so that at each junction 30 and 32 the susceptances of the bandpass and bandstop filters oppose and cancel over their respective frequency bands. Multiplexers of the form illustrated in prior art FIG. 1 are not found at the EHF band because lumped elements, i.e. inductors and capacitors, do not exist at those frequencies. Wide band commensurate line distributed filters have not been built in both bandpass and bandstop form.
A second type of multiplexer can be formed using bandpass filters without bandstop filters but requiring the use of 3dB hybrid couplers. One such example is illustrated in prior art FIG. 2. The triplexer 34 illustrated in FIG. 2 is comprised of three input 3dB couplers 36, 38 and 40, each of which feeds a pair of bandpass filters. Specifically, bandpass filters 42 and 44 pass filters in the frequency range f.sub.1 -f.sub.2, bandpass filters 46 and 48 pass frequencies in the band f.sub.2 -f.sub.3 and bandpass filters 50 and 52 pass frequencies in the range f.sub.3-f.sub.4 The outputs of each pair of bandpass filters are passed to another 3dB hybrid coupler, specifically, 54, 56 and 58 as illustrated. Briefly, this triplexer operates such that the input signal to 3dB hybrid coupler 36 is split at its output between the bandpass filters 42 and 44. The inputs to the bandpass filter 42 and 44 comprise one-half the power of the input signal to the 3dB coupler 36 and are 90 degrees out of phase with respect to each other. The second 3dB coupler 54 recombines the outputs of the bandpass filters 42 and 44 such that all of the power from the input signal to the 3dB coupler 36 within the frequency band f.sub.1 -f.sub.2 appears recombined, in-phase on the one output terminal of the 3dB coupler 54. The loaded terminal of the 3dB coupler 54 has essentially no output signal since the signals from the bandpass filters 42 and 44 are cancelled at that terminal due to a 180 degree phase shift therebetween. The fourth port 60 of 3dB coupler 36 receives all power from the input signal except that which was passed through the 3dB coupler 36 to the bandpass filters 42 and 44. Therefore, the output on the unloaded terminal of 3dB coupler 56 comprises all signals within the frequency band f.sub.2 -f.sub.3. Similarly, the output on the unloaded terminal of 3dB coupler 58 comprises all signals within the frequency band f.sub.3 -f.sub.4. The bandwidth of the form of multiplexer illustrated in FIG. 2 is dependent on the directivity and isolation of the hybrid couplers used. While attempts have been made to fabricate triplexers of the form illustrated in FIG. 2 in the frequency range over the 75-110GHz bandwidth, these attempts have been futile due to the poor performance of 3dB quadrature couplers over such a large frequency band.